1. Field of the Invention
This invention relates to an electrostatic reproducing apparatus capable of sensing the jamming of sheets of recording paper after image transfer, and the concentration of the toner.
2. Description of the Prior Art
Apparatus for electrostatic reproduction by an electrophotographic process such as electrophotographic copying machines and certain kinds of facsimile machines are well known. In an apparatus of this kind, an electrostatic image of a document or electrical data is formed on a photosensitive or dielectric member by the use of reflected light obtained by exposing and scanning the document or by optical data prepared on the basis of electrical data signals, and is then developed to form a visible image by a developer containing a toner and the resulting visible image is transferred to recording paper and is then fixed to provide a reproduced image. FIG. 1 shows the construction of an electrophotographic copying machine diagrammatically as an example of this type of electrostatic reproducing apparatus. The document G placed on a document glass table 1 is exposed by an exposure lamp 2 and the light reflected from the document G is projected onto a photosensitive member 5a on a rotary drum 5 via mirrors 3a and 3b and an exposure slit 4, forming an electrostatically charged image of the document G on the photosensitive member 5a. In proximity to the rotary drum 5 are disposed a charging electrode 6 for charging the photosensitive member 5a uniformly, a developing device 7, a transfer electrode 8, a separation electrode 9 which makes the recording paper easily separatable from the photosensitive member 5a after transfer, a separation pawl 10 for separating the recording paper from the photosensitive member 5a, a charge-eliminating electrode 11 for eliminating the charge remaining on the photosensitive member 5a, and a cleaning device 12 for removing any toner remaining on the photosensitive member 5a. Reference numeral 13 represents a jamming sensing means (hereinafter referred to as a "jamming detector") for detecting whether the recording paper is still being carried wound on the photosensitive member 5a because of a malfunction in the separation means described above, or the like, by the quantity of light reflected from the surface of the recording paper, anticipating the jamming of the paper in the cleaning device 12 and to prevent such jamming. A reflection type of photo-sensor is used conventionally as the jamming detector 13. The photo-sensor consists of a light-emitting diode emitting infrared light so that incoming external light does not influence it and the charging capacity of the photosensitive member is not reduced by it, and a photo-transistor receiving the reflected light and producing an equivalent electric signal.
The electrostatically charged image formed on the photosensitive member 5a is developed by the developing device 7 to produce a visible image, which is then transferred by the transfer electrode 8 to the recording paper P that is fed from a paper feed tray 14 by a paper feeder 15. After transfer, the recording paper P is separated from the photosensitive member 5a by the separation electrode 9 in cooperation with the separation pawl 10, is then transferred by a conveyor belt 16 and is heat-fixed by a fixing device 17. The paper P is finally ejected into a receiving tray 18.
If a two-component system developer consisting of a carrier such as iron powder and a colored resin powder used as the toner is used as the developer in the electrophotographic copying machines of the kind described above, the toner in the developer is used up during repeated copying so that the toner concentration and eventually, the density of the reproduced image, gradually drop. Hence, the toner must be replenished in order to keep the reproduced image density constant, and various methods of detecting the toner concentration have therefore been proposed in the past. As a typical example of such methods, a method of detecting the toner concentration by use of an optical sensor is known. According to this method, a plate 19 having a reference density (e.g., an optical reflection density of 1.0) is bonded to the end of the underside of the document glass table 1 as shown in FIG. 2 and this reference density plate 19 is exposed during the exposure step before the document is exposed. (In this drawing, the document table 1 moves in the direction indicated by an arrow.) The electrostatically charged image of the plate is formed on the photosensitive member 5a and is developed into a visible image so that the density of the visible image can be detected by an optical sensor (not shown) that is positioned in the proximity of the photosensitive member 5a.
A conventional sensor for detecting the toner concentration consists of a light emitting element such as a light emitting diode and a light receiving element such as a phototransistor, and infrared light is used as the detecting light in order to eliminate the influences of external light and to prevent a reduction in the charging capacity of the photosensitive member. It is also known to use this kind of sensor as a jamming detector.
From considerations of the processing system of an electrostatic reproducing apparatus using the electrophotographic process, it is preferable that a single sensor be used both to sense jamming of the paper and to detect the toner concentration because the disposition of two separate sensors results in an increase in the space they occupy and in the number of components. However, there is not a great difference between the sensor output level when a sheet of recording paper remains abnormally wound on the photosensitive member and the sensor output level under the normal condition where paper is not wound on the photosensitive member. Accordingly, if a single sensor is used to detect jamming as well as to detect the toner concentration, the detection is likely to become unreliable under various conditions if the jamming is sensed by judging whether or not the output level exceeds a threshold value. Another problem is that if the sensor is contaminated by toner, the sensor output level drops and jamming detection is no longer possible.
Various materials have been employed for the photosensitive member of electrophotographic copying machines of the kind described above, such as zinc oxide, amorphous selenium, OPC (organic semiconductors) and the like. It is known that if a photosensitive member made of one of these materials is exposed to light for an extended period of time, a phenomenon referred to as the "memory effect" occurs in which the chargeability of the exposed portion of the photosensitive member drops and an electric charge can not be easily induced in that portion. Since conventional electrophotographic copying machines make use of an infrared-light emitting diode as the jamming detection means, degradation of the chargeability of the photosensitive member is as obvious as when visible light is used, but because the light emitting diode is kept lit even during the period in which the reproduction is not being carried out, the surface potential of the photosensitive member is likely to become non-uniform when the apparatus has been in use for an extended period of time, and the quality of the reproduced image is likely to drop. If the light emitting element has directionality, the illumination of the radiated portion becomes so much greater that the degradation of chargeability becomes a critical problem if such a light emitting element is used to sense jamming. Moreover, fluorescent lamps having a reduced power consumption have been used in place of the conventional halogen lamp to save energy and this calls for a photosensitive material having a higher sensitivity. A higher sensitivity of the photosensitive material is also necessary in order to improve the reproducing speed. If the sensitivity of the photosensitive material is improved, the problem of the degradation of chargeability becomes greater.
To cope with these problems, a method has been proposed in which the light emitting element is lit not continuously but intermittently, and the proportion of time that the element is on to the time it is off is suitably adjusted. However, with this method too, the light emitting element must be turned on and off even during the period in which reproduction is not carried out, and a small area of the photosensitive member is undesirably illuminated for a long period when it is not rotating. In order to increase the difference of the output levels of the jamming detection means when jamming occurs and when it does not, a method which uses a visible-light emitting diode has also been proposed. If this method is applied to the above method in which the light emitting element is intermittently turned on and off, however, the problem of the chargeability of the photosensitive member can not be sufficiently solved.